The role of pyrroline-5-carboxylate (P5C) in nutrition-dependent intercellular communication and transmembrane signaling has been previously demonstrated. Our current studies have extended our previous findings and have elucidated the following mechanisms. 1) P5C was released into the culture medium by a number of cell lines. Glutamine and ornithine both could be precursors for the formation anA release of extracellular P5C but glutamine was the major source. Concentrations in conditioned medium reached 2-3 micrometers, a level which can mediate a number of effector functions. 2) We have identified a 50 kilodalton membrane protein which may be the specific membrane carrier protein for P5C. Previous work showed that the entry of P5C into cells is mediated by its own special carrier mechanism which transfers oxidizing potential pari passu with P5C entry. Using labeled p-chloromercuri-benzosulfonate, we have identified a band on SDS PAGE which shows the same sensitivity to specific inhibitors as does P5C uptake. 3) P5C stimulated thymidine, incorporation in cells activated by growth factors. We previously showed that PP- Rib-P production is markedly stimulated by P5C and that PDGF interacts synergistically with P5C. This synergism also applies to the incorporation of [3H ]thymidine. 4) The mechanism of these P5C effector functions involved the synergistic stimulation of phosphoinositides. P5C markedly increases the, production of inositol phosphates over that produced by growth factors. These studies further demonstrated that P5C serves as a mechanism for integrating nutritional influences with growth factor-stimulated cellular events.